The present invention relates to a method for variably actuating the valves of an internal combustion engine.
Historically, the efficiency, emissions, and performance of internal combustion engines have been adversely limited by fixed valve lift profiles, i.e., valve lift profiles wherein the timing of the opening and closing of the valves is fixed relative to the angular position of the engine crankshaft and the amount of lift imparted to the valves is also fixed. By fixing the valve lift profiles of the engine, inherent compromises were made between low-speed operation (idle) and high-speed operation for peak power. Importantly, engines having fixed valve lift profiles must incorporate a throttle device to control the airflow (and output) of the engine. Throttle devices introduce large throttling losses (pumping work) and reduce the efficiency of the engine. Further, throttled engines with fixed valve lift profiles also require exhaust gas recirculation (EGR) in order to control emissions of oxides of nitrogen (NOx) and exhibit levels of hydrocarbon (HC) emissions that exceed soon-to-be implemented limits during cold start operation.
In contrast, modern internal combustion engines may utilize one of several methods and/or devices to vary the valve lift profile to, at least in part, control the flow of gas and/or air into and/or out of the engine cylinders. Modern internal combustion engines may utilize such devices, such as, for example, variable valve actuating (VVA) mechanisms, continuously variable VVA mechanisms, two-step cam profile switching mechanisms, and valve deactivation mechanisms, to vary the amount by which the valves of an engine are lifted (i.e., opened). Furthermore, modern engines may utilize such devices, such as VVA mechanisms and cam phasers, to vary the timing of the opening and/or closing of the engine valves relative to the angular position of the engine crankshaft. Each of these approaches contributes incrementally to improvements in engine efficiency, and/or emissions reduction, and/or performance gains. However, each of those approaches has certain limitations and/or undesirable characteristics.
Continuously-variable VVA mechanisms provide substantially continuous variability in both the amount of valve lift and valve timing. However, continuously variable VVA mechanisms are typically relatively complex and expensive, and may require extensive redesign of the engine cylinder head for production implementation. Further, many continuously-variable VVA mechanisms have a limited maximum operational speed and thus may limit the peak performance or peak output of the engine relative to other VVA systems with higher maximum operational speeds.
Two-step cam profile switching mechanisms utilize a two-step cam profile switching device, such as, for example, a two-step roller finger follower (RFF), to switch between two discrete valve lift profiles depending at least in part upon engine operating conditions and/or parameters. Two-step systems are relatively simple and are operable over a relatively wide range of engine operating speeds. Further such systems are relatively easy to package on new and even existing engines. By operating the two-step cam profile switching mechanism in conjunction with a cam phaser a wide range of variation in the valve lift characteristic is obtained. The valve lift profile is selected via the two-step cam profile-switching device, and the timing of the valve lift is adjusted and/or varied by the cam phaser. Although two-step systems achieve a relatively wide range of variation in the valve lift profile, they nonetheless represent a tradeoff between the mechanical simplicity and less-than-continuous variability they provide relative to the mechanical complexity yet full variability that a continuously-variable VVA mechanism provides. Two-Step VVA systems also impose some special requirements on cam phasers, and may require both faster cam phaser response rates and greater cam phaser authority to achieve the full potential of these systems.
Therefore, what is needed in the art is a variable valve actuation system that enables an increased and relatively wide range of variation of the valve lift profiles, and yet is relatively simple.
Furthermore, what is needed in the art is a valve actuation system that provides an increased and relatively wide range of variation of the valve lift profiles over a relatively wide range of engine operating speeds.
Still further, what is needed in the art is a method of valve actuation that achieves an increased and relatively wide range of variation of the valve lift profiles across a relatively wide range of engine operating speeds and yet does so with relative simplicity.
Moreover, what is needed is a method of valve actuation that achieves an increased and relatively wide range of variation of the valve lift profiles, and does so with conventional cam phasers having both conventional cam phaser rates and ranges of authority.
The present invention provides a method of variably actuating a valve of an engine. The method includes selecting one of three valve lift profiles dependent at least in part upon engine operating conditions and parameters. The selected valve lift profile is phased relative to the angular position of the engine crankshaft dependent at least in part upon engine operating conditions and parameters. The valve is actuated according to the selected and phased valve lift profile.
An advantage of the present invention is that an increased range of variation of the valve lift profile is achieved with relative mechanical simplicity.
Another advantage of the present invention is that an increased range of variation of the valve lift profile is achieved across an increased range of engine operating speeds.
Yet another advantage of the present invention is that full potential of the system is achieved without requiring increased phaser rates and without increased phaser authority.